Plasma arc torches are commonly used for cutting, welding and spray bonding of workpieces and are operated by directing a plasma consisting of ionized gas particles toward a workpiece. In the operation of a typical plasma arc torch, a gas to be ionized is supplied to the front end of the plasma arc torch and channeled between a pair of electrodes before exiting through an orifice in the torch tip. One electrode, which is at a relatively negative potential, is usually referred to as the "cathode" or simply as the "electrode". The torch nozzle, which is adjacent to the end of the "electrode" at the front end of the torch, constitutes the relatively positive potential electrode or "anode".
When a sufficiently high voltage is applied, an arc is caused to jump the gap between the electrode and the torch housing, thereby heating passing gas around the electrode and between the electrode and housing and causing it to ionize. A pilot, pulsating voltage between the electrode and the housing starts the plasma arc. The ionized gas flows out of the torch and appears as an arc that extends externally from the outlet in the torch nozzle. In the normal transferred arc operation, the workpiece serves as the anode. This operation is initiated by the torch head being moved close to the workpiece so the arc jumps or transfers between the electrode and the workpiece.
In conventional torches, the charged electrode is typically made of copper with a tungsten electrode insert and current flows between the tungsten insert and the torch tip or workpiece when the torch is operated. Tungsten is oxidized easily at high temperatures so that if the gas to be ionized is air, the tungsten insert becomes oxidized and is rapidly consumed, thus necessitating frequent replacement. The gas to be used for creating the plasma is typically an inert gas, such as nitrogen or argon, in order to reduce oxidation and thereby prolong electrode life. Where air is used, materials resistant to oxidation such as hafnium or zirconium have been used as the electrode insert material. Examples of inserts in insert bores of electrode members are illustrated in U.S. Pat. Nos. 4,581,516; 4,691,094 and 4,959,520.
Typically, the insert is disposed in a closed insert bore machined into one end of the electrode body. The machining of the insert bore is accomplished in a time efficient and therefore cost effective manner by bathing the bore producing machinery with a constant stream of cutting oil. The problem which the present invention solves is caused by the cutting oil from the machining process remaining in the insert bore and being evaporated into a high pressure vapor trapped between the insert and the bore. More specifically, cutting oil is trapped in the cavity formed between the closed end of the insert bore and the end of the insert facing and in space relation thereto.
When the electrode is heated as the arc is established from the electrode, the pressure of the vapor generated by the evaporation off of the cutting oil in the cavity between the closed end of the insert bore and the facing end of the insert can be sufficient to push or move the insert at least partially out of the electrode body, typically in the first few seconds of operation.
A minimum force of about 17 pounds and a maximum force of about 85 pounds have proven sufficient to push an insert out of an insert bore. Based on a cylindrical insert having a 0.06 inch diameter and an area of 0.00283 square inches, this translates to a minimum pressure of 6,012 pounds per square inch (psi) and a maximum pressure of 35,714 psi. Since the operating temperature of an electrode body at the end containing the insert is between about 400.degree. centigrade (C) and 600.degree. C., a petroleum based cutting oil, which will certainly boil at even lower temperatures, will form a vapor. This oil based vapor can reach a considerable pressure of about 6,000 psi or more and thereby force the insert out of the electrode insert bore.
To overcome this problem, a number of possible solutions were considered but ultimately rejected for financial or operational considerations.
One solution involves eliminating the use of cutting oil in machining the insert bore into the electrode body. However, conventional machinery which accurately and expeditiously bores the electrode member requires a constant shower of oil to insure the integrity of the bearing life and, therefor, its accuracy. Although the manufacturing can be accomplished without oil, the process takes longer and is therefore more expensive.
A second solution to the problem is to extend and connect the insert bore to an inlet cavity extending from the other end of the electrode body so that the oil can drain therein. Examples of this configuration are disclosed in U.S. Pat. Nos. 3,450,926; 4,590,354; 4,864,097 and 4,967,055. However, the reduction in the mass of the electrode body significantly reduces the mass required for adequate heat transfer and therefore the operational life of the electrode will be shortened.
A third solution is to provide a narrow connection passage from the insert bore to the inlet cavity, such as disclosed in U.S. Pat. No. 4,748,312, whereby the trapped oil can escape. This solution would substantially eliminate the heat transfer problem but leaves a deep, narrow channel in which trapped oil and plating residue can collect. Clearing and probably baking operations would then be required to carefully clear this channel. Otherwise, any remaining debris would be free to migrate back into the torch body and potentially cause electrical tracking or clog the plasma gas swirl holes.